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Background Ustekinumab (UST), a human monoclonal antibody that binds the p40 subunit of interleukin 12 (IL-12) and IL-23, is licensed for induction and maintenance therapy of moderate to severe inflammatory bowel disease (IBD). To date, there is limited data published on any potential association between ustekinumab serum trough levels and mucosal healing in order to guide treatment strategies and appropriate dosing. Aim This study aims to identify a relationship between maintenance ustekinumab serum trough levels and mucosal healing and/or response in patients with Crohn’s disease in an observational cohort study. Methods Ustekinumab serum trough levels and antibody titres were analyzed in patients on maintenance drug using an ELISA drug-tolerant assay. Mucosal response (MR) was defined as ≥50% reduction in fecal calprotectin level (FC) and/or ≥50% reduction in the Simple Endoscopic Score for Crohn’s Disease (SES-CD score). Mucosal healing (MH) was defined as FC ≤150 µg/mL and/or global SES-CD score ≤5. Median trough levels were analyzed using the Kruskal-Wallis test, and logistic regression was used to determine sensitivity and specificity of levels predicting mucosal response. Results Forty-seven patients on maintenance ustekinumab for Crohn’s disease were included in this study. The majority were female (66%), with a median age of 40 years (21-78 years). The majority of patients were biologic-experienced (89.4%, n = 42). Patients with histologically confirmed Crohn’s disease represented 100% (n = 47) of the cohort. Over one-third of patients (n = 18, 38.3%) were on higher than standard dosing of 90 mg every 8 weeks. Patients with mucosal healing (n = 30) had significantly higher mean serum ustekinumab levels (5.7 µg/mL, SD 6.4) compared with those with no response (1.1 µg/mL, SD 0.52; n = 7, P < .0001). A serum ustekinumab trough level greater than 2.3 µg/mL was associated with MH, with a sensitivity of 100% and specificity of 90.6% (likelihood ratio 10.7). Similarly, for patients with MR (n = 40), we observed a higher mean serum ustekinumab trough level (5.1 µg/mL, SD 6.1) compared with those with no response (1.1 µg/mL, SD 0.52; n = 7, P < .0001). Furthermore, a serum ustekinumab trough level greater than 2.3 µg/mL was associated with a 10-fold increased likelihood of mucosal response vs mucosal nonresponse (sensitivity 100%, specificity 90.5%, likelihood ratio 10.5). Conclusion This study demonstrates that higher ustekinumab serum trough levels are associated with a greater likelihood of achieving mucosal healing and mucosal response in patients with Crohn’s disease regardless of prior biologic exposure. Further prospective studies are required to correlate target maintenance trough levels and the optimal time to dose-escalate in order to improve patient outcomes.

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